This invention relates in general to sewing machines and in particular to a new and useful stitch group sewing machine which includes means for varying the stitch length of each stitch group.
A stitch group sewing machine wherein the length of the stitches in the group is controlled by gearing is shown in German Pat. No. 824,738 (U.S. Pat. No. 2,411,493). This known stitch group sewing machine is a buttonhole sewing machine with a work holder gripping the work in the region of the buttonhole to be formed. The work holder is driven by a first gearing controlled cam plate which makes one complete revolution during the formation of a buttonhole. A first cam slot moves the work holder parallel to the longitudinal axis of the sewing machine, and hence in lengthwise direction of the buttonhole to be formed. A second cam slot moves the work holder crosswise to the longitudinal axis of the sewing machine in connection with a second gearing. The needle bar is mounted in a pivotably mounted frame and executes swinging movements transverse to the longitudinal axis of the sewing machine for the formation of zig-zag stiches.
The first gearing contains an angle lever, whose one leg carries a sensor engaging in the cam slot and whose other leg comprises a link guide. In the link guide a sliding block of a pitman connected with the work holder engages in the link guide. By displacement of the sliding block in the link guide the transmission of the bearing and hence the amount of forward movement of the work holder parallel to the longitudinal axis of the sewing machine, can be varied and in this way the length of the buttonhole to be formed can be adjusted.
The cam plate is intermittently set into rotation by a one way clutch which is in drive connection with the main shaft of the sewing machine via a drive mechanism. The transmission ratio of the drive mechanism can be adjusted similary as for the first gearing, whereby the speed of rotation of the cam plate can be varied at constant rotational speed of the main shaft. In this manner the number of stitches which form the buttonhole can be varied.
If when changing the buttonhole length the number of stitches is to be varied as well to obtain a constant stitch length, the transmission ratio of the drive mechanism must be matched very exactly to the transmission ratio of the first gearing. This mutual adaptation or adjustment is time consuming and requires a certain skill and experience, so that generally it must be effected by a mechanic rather than by the seamstress.
To eliminate the danger of follow-up or continued running that exists in the operation of a one way clutch due to inertia, the known sewing machine is provided with an intermittently active brake device acting at the circumference of the cam plate. Since such a brake is subject to wear, and since the braking depends on the surface quality or the degree of soiling of the friction surfaces so that no assurance for smooth operation of the cam plate is given especially at high speeds, such a drive system is not suitable for modern sewing machines, for which generally high speeds are required, to reduce the sewing time.